It is known in prior art to provide an NOx trap catalyst as an exhaust gas purification catalyst in an exhaust passage of lean-burn internal combustion engines that operate at lean air-fuel ratios higher than the theoretical air-fuel ratio. The NOx trap catalyst has the function of storing NOx in the exhaust gas when the air-fuel ratio of the exhaust gas is a lean air fuel ratio. It should be understood that the expression “storing NOx” (along with its derivatives) used in this specification also means adsorbing NOx. As the NOx trap catalyst as such, an NOx storage reduction catalyst (which will also be referred to as “NSR catalyst” hereinafter) is used in some cases. The NSR catalyst has the function of storing NOx in the exhaust gas when the exhaust gas has a lean air-fuel ratio and reducing NOx stored therein when the exhaust gas has an air-fuel ratio equal to or lower than the theoretical air-fuel ratio and a reducing agent is present. It should be noted that the NOx trap catalysts also include catalysts that have the function of storing NOx in the exhaust gas but do not have the function of reducing NOx stored therein.
Patent Literatures 1 to 3 disclose technologies relating to assessing deterioration of an NSR catalyst in systems in which the NSR catalyst is provided as an NOx trap catalyst in an exhaust passage of an internal combustion engine. Patent Literature 1 discloses a system including an NOx sensor provided in the exhaust passage downstream of an NSR catalyst. The NOx sensor has the property of sensing not only NOx in the exhaust gas but also NH3. When a reducing agent (HC) is supplied to the NSR catalyst in which NOx is stored, the reducing agent and NOx react to produce NH3. The amount of NH3 thus produced depends on the amount of NOx stored in the NSR catalyst. The NOx storage capability of the NSR catalyst may decrease as it stores SOx in the exhaust gas. This is called SOx poisoning. If the NSR catalyst deteriorates due to increases in the amount of SOx deposited in a state in which recovery from the poisoning is impossible, the amount of NOx stored in the NSR catalyst becomes smaller than that in the NSR catalyst in a normal condition. In the technology disclosed in Patent Literature 1, the deterioration of the NSR catalyst is assessed on the basis of the measurement value of the NOx sensor when reducing agent is supplied to the NSR catalyst by making the air-fuel ratio of the exhaust gas rich. If the amount of NOx stored in the NSR catalyst is smaller due to deterioration of the NSR catalyst as described above, the quantity of NH3 produced with the supply of reducing agent to the NSR catalyst is smaller. In consequence, the quantity (or the concentration) of NH3 measured by the NOx sensor is smaller than that in the case where the NSR catalyst is in a normal condition. Therefore, it is possible to assess the deterioration of the NSR catalyst on the basis of the measurement value of the NOx sensor at the time when reducing agent is supplied to the NSR catalyst.
Patent Literature 2 describes that when the process of recovery from SOx poisoning of the NSR catalyst (namely the process of desorbing SOx stored in the NSR catalyst from it) or the process of regenerating a filter provided in the exhaust passage upstream of the NSR catalyst (namely the process of removing particulate matter trapped in the filter by combustion) is performed, the temperature of the NSR catalyst becomes high to cause desorption of NOx stored in the NSR catalyst (high-temperature regeneration). In the technology disclosed in Patent Literature 2, the deterioration of the NSR catalyst is assessed on the basis of the integrated amount of NOx stored into the NSR catalyst during the period after the desorption of NOx from the NSR catalyst by the high-temperature regeneration until the NOx storage capacity of the NSR catalyst is saturated.
Patent Literature 3 discloses a technology of assessing the deterioration of an NSR catalyst on the basis of the degree of change in the removal rate, which represents the difference in the NOx removal rate between before and after the NOx reduction process, namely the process of reducing NOx stored in the NSR catalyst.
Patent Literature 4 discloses a technology of correcting the length of the interval between the end of the NOx reduction process and the start of the next time NOx reduction process on the basis of the degree of deterioration of the NSR catalyst in an exhaust gas purification apparatus including an NSR catalyst.